The invention relates to making parts out of composite material having a matrix that is ceramic or at least partially ceramic, and referred to below as a CMC material.
The field of application of the invention is making parts that are to be exposed in operation to high temperatures in an oxidizing atmosphere, and in particular in the fields of aviation and space, specifically parts for the hot portions of aviation turbine engines, it being understood that the invention may be applied in other fields, for example in the field of industrial gas turbines.
CMC materials possess good thermostructural properties, i.e. strong mechanical properties that make them suitable for constituting structural parts, together with the ability to conserve those properties at high temperatures.
The use of CMC materials instead of metal materials for parts that are exposed in operation to high temperatures has therefore been recommended, particularly since CMC materials present density that is substantially smaller than that of the metal materials they replace.
A well-known method of fabricating parts out of CMC material comprises making a fiber preform from a woven fiber texture, the preform being consolidated and densified by a ceramic matrix by chemical vapor infiltration (CVI). By way of example, reference may be made to Documents WO 2010/066140, WO 2010/116066, and WO 2011/080443, which relate to fabricating parts for aviation turbine engines out of CMC material.
A second well-known method consists in making a preform from fiber plies in which silicon carbide based fibers are coated by CVI in a layer of boron nitride BN covered in a layer of carbon or carbide, and in particular a layer of silicon carbide SiC. The fiber plies are preimpregnated with a composition containing a carbon powder or ceramic powder and an organic binder, or in a variant they are impregnated with such a composition after the preform has been formed. Once the binder has been eliminated, densification is performed by infiltrating molten silicon, possibly enriched with boron, where such a densification process is known as a melt infiltration (MI) process. By way of example, reference may be made to Documents U.S. Pat. No. 4,889,686, U.S. Pat. No. 4,944,904, or U.S. Pat. No. 5,015,540. In well-known manner, the BN interphase material coating the fibers performs an embrittlement-relief function in the composite material, while isolating the fibers from the molten silicon during the MI process.
The CVI densification process enables matrix phases to be obtained that are of uniform thickness and of controlled composition, in particular matrix phases made of materials having specific functions of diverting cracks or of being self-healing, thereby imparting long lifetime at high temperatures and under an oxidizing atmosphere. The term “self-healing” is used herein to designate a material that, in the presence of oxygen, forms a vitreous composition that is capable of passing to the pasty or fluid state in certain temperature ranges, thereby sealing cracks that appear within the material. Examples of self-healing materials are an Si—B—C ternary system or a boron carbide system capable of forming a borosilicate type glass. Nevertheless, CVI densification processes are relatively lengthy and expensive.
Conversely, densification by an MI process does not enable matrix phases to be formed with controlled thickness and composition, however it is much faster and easier to perform than CVI densification, and from this point of view it can be attractive. Nevertheless, in the presence of cracking, parts obtained by the second above-mentioned method present a lifetime that is short in an oxidizing atmosphere as from temperatures of about 800° C., because the BN material of the interphase coating oxidizes into B2O3 which, in the presence of moisture, forms a volatile oxide leading to BN material depletion, and thus to the embrittlement-relief function disappearing progressively. Unfortunately, it is practically inevitable that CMC materials will be subjected to cracking because of the thermal cycling to which they are subjected in operation. Furthermore, forming BN by CVI is complex and makes use of precursor gases of the NH3 and BCl3 or BF3 type, thus requiring an installation that is complex, in particular in order to process effluents on an industrial scale. Furthermore, depositing a layer of SiC on the BN material of the interphase coating can be of limited effectiveness in protecting the coating while molten silicon is being infiltrated, since an SiC deposit obtained by CVI generally presents a columnar structure into which the molten silicon can become infiltrated.